The bipolar transistor is an electronic device with two pn junctions in very close proximity. There are three device regions: an emitter region, a base region, and a collector region. The two pn-junctions are known as the emitter-base junction and the collector-base junction. Modulation of the current in one pn-junction by means of a change in the bias of the other nearby pn-junction is called bipolar-transistor action. Because the mobility of minority carriers (electrons) in the base region of npn transistors is higher than that of holes in the base of pnp transistors, higher frequency operation and higher speed performances can be obtained with npn devices. For this reason, the following discussion is generally in the terms of npn transistors, but it is to be understood that the discussion is applicable to pnp transistors as well.
The desired device characteristics of bipolar transistors include: high current gain, high frequency ac operation, fast switching speed, high device-breakdown voltages, minimum device size, and high reliability of device operation.
In order for a semiconductor device to operate at a high voltage and at a high frequency, the doping concentrations of the various pn-junctions must be carefully controlled to provide depletion regions so that the device has the necessary breakdown voltage. As is known in the semiconductor art, there is a tradeoff between high operating voltages and high frequency operation. For example, to obtain a high breakdown voltage, the doping concentrations on each side of a pn-junction should be low, however, a low doping concentration increases the resistance of the device and will decrease the operating frequency.
To reduce this resistance, underneath a collector region with low doping concentrations may be a subcollector region that is heavily doped the same type as the collector. The subcollector provides a low resistance path from the active part of the transistor to a collector contact.
The manufacture of high performance bipolar transistors requires the reduction of the vertical profile of the device as well as the reduction of transistor parasitics. In order to reduce the collector-to-emitter transit time of the carriers, it is preferable to position the subcollector close to the collector-base junction. For example, to achieve 200 GHz fT, a thin collector (normally made by growing an epitaxial layer) must be used. However, no matter what selectively implanted collector profile is used, as the vertical profile is scaled, electric fields between the base and subcollector increase, resulting in higher avalanche multiplication and eventually to a lower collector-emitter breakdown voltage.
Ultimately, the maximum attainable breakdown voltage is a function of the distance between the base and the subcollector beneath it.